1. Field of the Invention
The invention relates to a method of forming thin film passive integrated optical components by proton exchange of an optically transparent substrate in an acid bath followed by annealing to extend and stabilize regions of increased refractive index.
2. Description of the Prior Art
In a paper by Stewart B. Miller, Integrated Optics: An Introduction, BSTJ 48, 2059 (1969), it was suggested that dielectric waveguides could be formed in a substrate having a region of increased refractive index surrounded by a region of normal refractive index, with a difference in refractive of only 0.01. It was further suggested that combinations of optical elements could be formed in a planar waveguide using photolithographic techniques to provide integrated optical circuits.
Attempts to fabricate such structures were made by diffusion of titarium into a lithium niobate substrate. However, such structures with lithium niobate crystals are susceptable to optical damage and are therefore limited in dynamic range, since only low-power excitation beams may be applied. The index change with lithium niobate is of the order of 0.01, which requires a relatively long waveguide path and hence longer optical components. Moreover, the diffused guide structures support multimode polarization, and hence are unsuited where polarization control is required, as in low-loss optical switches with low crosstalk.
In the fabrication of waveguides and thin film optical components by localized modification of the refractive index of an optically transparent substrate, it has been demonstrated that a process using proton exchange in benzoic acid has been effective in forming optical waveguides in a lithium niobate substrate. The technique consists in emersing x-cut or z-cut LiNbO.sub.3 substrates into a molten bath of benzoic acid at a fixed temperature between the melting point and boiling point of the benzoic acid for a predetermined period of time which varies according to the required thickness for the guiding layer and the desired ultimate refractive index profile. The waveguides so obtained have been found to have a step-like index profile with a maximum increase in the extraordinary index n.sub.E of 0.12 at .lambda.=633 nm, no change being observed in the ordinary index n.sub.O. Such devices offer advantages over similar structures fabricated by the diffusion of titanium, in that their fabrication is quicker and more economical, provides a greater index change .DELTA.n.sub.E, and can be adapted to single-mode propagation. In consequence, optical devices, such as waveguides with bends of short radii, lenses, and reflectors which are impractical with the diffused structures, become realizable with proton exchange. Annealing can be used to convert the step-index profile obtained with proton exchange to a parabolic shaped gradient-index profile having a reduced value of n.sub.E at the substrate surface but providing an increased guide depth for matching to fiber-optic cables. However, the proton-exchanged LiNbO.sub.3 structures have suffered from instability, showing significant changes in refractive index in periods as short as one day over several months.
Single-crystal LiTaO.sub.3 is, in many respects, a more attractive substrate material for integrated-optic applications than LiNbO.sub.3. A comparison of LiTaO.sub.3 with LiNbO.sub.3 reveals that LiTaO.sub.3 is less susceptible to optical damage, has a smaller birefringence, is harder, and is easier to polish. The electro-optic coefficients of the two materials are essentially the same. Single-crystal LiTaO.sub.3 is a stable, pure, highresistivity material, available in high optical quality. Yet, in the past, the diffusion of metal ions (such as Ti.sup.+) into LiTaO.sub.3 to create optical waveguides has been "thermally forbidden" because the required diffusion temperatures are higher than the Curie temperature of LiTaO.sub.3. This is not a problem with the proton exchange process due to the relatively low temperatures at which it occurs (&lt;250.degree. C.). Heretofore, extending the process to produce waveguides in LiTaO.sub.3 has not been demonstrated.